A/D CONVERTER

PURPOSE:To convert a voltage to be converted into a digital data with simple constitution by measuring the time after a discharge path of a capacitor of a time constant circuit is interrupted till the said capacitor is charged into the voltage to be converted with a reference voltage. CONSTITUTION:The level of a control terminal SW is inverted to an L level at the start of measurement, a transistor(TR) 13 is turned off to interrupt the discharge path of a capacitor 12. Simultaneously, a computer 14 starts counting the clock signal. On the other hand, when the TR 13 is turned off, the capacitor 12 is charged at a prescribed time constant depending on resistors 10, 11 and the capacitor 12 by a reference voltage Vref at a voltage terminal 8. When the charging voltage of the capacitor 12 applied to a noninverting input terminal (-) of the comparator 7 reaches the voltage to be converted at an input terminal 6, the output signal at the comparator 7 is inverted to an L level, the level at an input terminal (i) is inverted to an L level, the count is stopped and the count is transferred to an arithmetic means of the computer 14. Then the count transferred by the arithmetic means is converted into a voltage and the voltage to be converted is calculated.